In an oxidizing environment, there are substances that under suitable conditions, take up electrons and become reduced. These electrons typically come from the atoms of metal objects exposed to the oxidizing environment An oxidizing environment is characterized by the presence of at least one chemical, the atoms of which, in that environment, are capable of being reduced by acquiring at least one electron from the atoms of the metal. In “donating” an electron, the metal becomes oxidized. As the process of oxidation continues, a metal object becomes degraded to the point that it can no longer be used for its intended purpose.
On land, oxidation is prevalent in, among other things, bridges and vehicles, when they are exposed to salt that is spread on roads to prevent the formation of ice in cold climates. The salt melts the snow and ice and, in so doing, forms an aqueous salt solution. The iron or steel in the bridges or vehicles, when exposed to the salt solution, is readily oxidized. The first visible sign of oxidation is the appearance of rust on the surface of the metal object. Continued oxidation leads to the weakening of the structural integrity of metal objects. If the oxidation is allowed to continue, the metal object rusts through and eventually disintegrates or, in the case of the metal in bridges, becomes too weak to sustain the load to which it is subjected. The situation has become worse in recent years with increased concentrations of pollutants and the demand for lighter, more fuel-efficient vehicles requiring thinner sheet metal and the abandonment of mainframe construction.
An aqueous salt solution is also the cause of corrosion in a marine environment and is responsible for the oxidation of hulls of ships, offshore pipelines, and drilling and production platforms used by the oil industry.
Early methods of corrosion prevention relied on applying a protective coating, for example of paint, to the metal object. This prevents the metal from coming in contact with the oxidizing environment and thereby prevents corrosion. Over a long time, however, the protective coating wears off and the process of oxidation of the metal can begin. The only way to prevent oxidation from starting is to reapply the coating. This can be an expensive process in the best of circumstances: it is much easier to thoroughly coat the parts of an automobile in a factory, before assembly, than to reapply the coating on an assembled automobile. In other circumstances, e.g., on an offshore pipeline, the process of reapplying a coating is impossible.
Other methods of prevention of oxidation include cathodic protection systems. In these, the metal object to be protected is made the cathode of an electrical circuit. The metal object to be protected and an anode are connected to a source of electrical energy, the electrical circuit being completed from the anode to the cathode through the aqueous solution. The flow of electrons provides the necessary source of electrons to the substances in the aqueous solution that normally cause oxidation, thereby reducing the “donation” of electrons coming from the atoms of the protected metal (cathode).
The invention of Byrne (U.S. Pat. No. 3,242,064) teaches a cathodic protection system in which pulses of direct current (DC) are supplied to the metal surface to be protected, such as the hull of a ship. The duty cycle of the pulses is changed in response to varying conditions of the water surrounding the hull of the ship. The invention of Kipps (U.S. Pat. No. 3,692,650) discloses a cathodic protection system applicable to well casings and pipelines buried in conductive soils, the inner surfaces of tanks that contain corrosive substances and submerged portions of structures. The system uses a short pulsed DC voltage and a continuous direct current.
The cathodic protection systems of the prior art are not completely effective even for objects or structures immersed in a conductive medium such as sea water. The reason for this is that due to local variations in the shape of the structure being protected and to concentrations of the oxidizing substances in the aqueous environment, local “hot spots” of corrosion develop are not adequately protected and, eventually, cause a breakdown of the structure. Cathodic protection systems are of little use in protecting metal objects that are not at least partially submerged in a conductive medium, such as sea water or conductive soil. As a result, metal girders of bridges and the body of automobiles can not be effectively protected by these cathodic systems.
Cowatch (U.S. Pat. No. 4,767,512) teaches a method aimed at preventing corrosion of objects that are not submerged in a conductive medium. An electric current is impressed into the metal object by treating the metal object as the negative plate of a capacitor. This is achieved by a capacitive coupling between the metal object and a means for providing pulses of direct current. The metal object to be protected and the means for providing pulses of direct current have a common ground. In his preferred embodiment, Cowatch discloses a device in which a DC voltage of 5,000 to 6,000 volts is applied to the positive plate of a capacitor separated from the metal object by a dielectric. Small, high frequency (1 kilohertz) pulses of DC voltage are superimposed on the steady DC voltage. Cowatch also refers to a puncture voltage of the dielectric material as about 10 kV.
Because of the safety hazards of having the high voltage applied at a place that exposes humans and animals to possible contact with the metal object or any other part of the capacitive coupling, Cowatch requires limitations on the maximum energy output of the invention.
Cowatch discloses a two-stage device for obtaining the pulsed DC voltage. The first stage provides outputs of a higher voltage AC and a lower voltage AC. In the second stage, the two AC voltages are rectified to give a high voltage DC with a superimposed DC pulse. Cowatch uses at least two transformers, one of which may be a push/pull saturated core transformer. Because of the use of transformers, the energy losses associated with the invention are high. Based on the disclosed values in Cowatch, the efficiency can be very low (less than 10%). The high heat dissipation may also require a method of dissipating the heat. In addition, the invention requires a separate means for shutting off the device during prolonged periods of non-use to avoid discharging the battery.
A somewhat related problem that affects submerged structures is caused by the growth of organisms. Mussels, for example, are a serious problem with municipal water supply systems and power plants. Because of their prolific growth, they clog the water intakes required for the proper operation of the water supply system or the power plant, causing a reduction in the flow of water. Expensive cleaning operations have to be carried out periodically. Barnacles and other organisms are well known for fouling the hulls of ships and other submerged parts of structures. Conventional means of dealing with this include the use of antifouling paints and thorough cleaning at regular intervals. The paints may have undesirable environmental effects while the cleaning is an expensive process, requiring that the ship be taken out of commission while the cleaning is done. Neither of these is effective in the long run.
It is a goal of the present invention to provide corrosion protection to metal objects even when the objects to be protected are not immersed in an electrolyte. It is a further goal of the present invention to accomplish this without exposing humans or animals to the risk of high voltages. In addition, the device should also be energy efficient, thereby reducing the drain on the power source and should not require any special means for heat dissipation. It also should, as part of the circuitry, have a battery voltage monitor that shuts off the pulse amplifier if the battery voltage drops below a predetermined threshold, thus conserving battery power. This is particularly useful because cold weather conditions under which corrosion is more likely due to exposure to salt used to melt ice on roadways, also imposes greater demands on a battery for starting a vehicle. In addition to cold weather, high temperatures and humidity also lead to increased corrosion simultaneously with increased demands on battery power for starting a vehicle. It is also a goal of the present invention to inhibit the growth of organisms on submerged structures. Finally, it is also a goal of the present invention to protect the circuitry from damage if the apparatus is inadvertently connected to the battery with reversed polarity.
It is, therefore, desirable to provide an improved control for corrosion protection.